Move LND node communication to background script

[wbobeirne]

Indirect progress towards #46, #42, #78. Should unblock #222.

Description

This refactors the lib/lnd-http library to have two API-compatible implementations

1. The good ol' LndHttpClient is now run in the background script
2. All other contexts (popup, options, content script) will now use an LndMessageClient, that has an identical API, but just sends messages to the background script to send.

This is an extremely ugly layer of abstraction, not to mention the code is nearly sinful to avoid too much code duplication, but it comes with some major benefits:

• HTTP requests are now happening in their own thread, which should avoid most common tactics for trying to snoop on requests and responses
• LND interactions in the content script are no longer subject to the limitations of doing things in the content context (CORS errors, CSPs of the page you're on etc.)

Steps to Test

Really the whole app, unfortunately